The present invention relates to an in-plane switching (In-plane switching: IPS) system liquid crystal display apparatus in which the electric field is applied to a liquid crystal layer in substantially parallel direction against a substrate and a liquid crystal display apparatus is operated, in particular relates to a large size panel use structure having a good productivity of an alignment control layer.
A strong need for a large size, a wide viewing angle and a multi-color performance of a screen size of a liquid crystal display apparatus is heightened. As to a viewing angle characteristic necessary for the large size, many definitions exists which are as a range in which a half-tone level is not reversed, a range in which a brightness, a contrast ratio, and a color tone are not varied, etc.. In the multi-color display, it is necessary to heighten a respective color re-production and further it is necessary to improve drastically the viewing angle characteristic according to the above stated definitions. As the liquid crystal display being compatible with the wide viewing angle and the large screen, it has proposed a combination system (IPS-TFT-LCD) in which IPS liquid crystal and a thin layer transistor (Thin layer transistor TFT) (Oota et al, Proceeding of the Fifteenth International Display Research Conference (Asia Display ""95; p.707) and a practical use of a monitor system in which a display screen size has diagonal 13.3 inches (corresponded to 15 type CRT) has started (Kondo et al, SID ""96 Digest No.8.1).
However, to realize the practical use of IPS-TFT-LCD corresponded to a screen larger than more than 17 type which will be a main current in CRT after this, it is necessary to develop a large size panel use new structure and process. In the conventional technique, it does not refer to a method for adding a homogeneous alignment characteristic in a panel having a stepped difference structure which is a subject mater of the present invention.
In IPS-TFT-LCD, it accompanies the difficulty of an alignment processing. A margin of this is narrow remarkably in comparison with a conventional type TN (Twisted Nematic) system, in particular in normally open type TN system (at a low voltage side a bright display, at a high voltage side a dark display). The reasons for the narrow margin are three points of following items (1)-(3).
(1) Stepped Difference Structure
In IPS-TFT-LCD, in principle it is necessary to arrange many long and narrow electrodes (at the case may be, it is called as an inter digital electrode) having several microns order degree.
Accordingly, a fine stepped difference structure is formed. A largeness of the stepped difference is determined by a thickness of the electrodes and shapes of various kinds of layers which are formed on the electrodes, in ordinary it has more than 0.1 micron. At the most upper layer of these layers, a high molecular layer such as polyimide is formed as an alignment control layer (it is called as an alignment layer). In the conventional mass-production technique, a surface of this alignment control layer is carried out according to a rubbing processing and a liquid crystal alignment ability is added. On the other hand, the rubbing use cloth is constituted by binding narrow fibers having a thickness of 10-30 microns degree, substantially since every one narrow fiber gives the sharing force in a predetermined direction to a local portion of the alignment layer, a processing for adding the liquid crystal alignment ability is carried out. As the fibers, there is an extremely narrow fiber having several microns degree, however as the rubbing use fiber, however since the rigidity for adding the some degree friction force is required, it has not used practically. Since an electrode interval in IPS system is 10-30 micron degree same to that of a diameter of the above stated fiber, the rubbing at a vicinity of the stepped difference is not carried out fully, as a result the alignment is disturbed easily. The disturbance in the alignment causes a lowering in a picture quality such as a rise of a black level and a lowering in the contrast ratio according to the above stated rise, and a non-homogeneity in the brightness.
As a method for solving the above stated problems, it has proposed a method for flattening a surface of the alignment layer, however the complete flattening manner invites following the side effects and this method is not used practically. A first is a problem which is caused by a phenomenon in which a spacer for controlling a liquid crystal layer at constant is moved easily. By the move of the spacer, a spacer distribution becomes up homogeneously and a liquid crystal layer thickness becomes non-homogeneously, accordingly the non-homogeneity of the brightness is invited. Further, during the spacer move the surface of the alignment layer is hurt and it causes a light leakage. From these points, it is necessary to have some degree stepped difference. Further, to solve the problem about the spacer move, it is desirable to form the stepped difference on one of a pair of substrates, in a case where TFT side is flattened and the stepped difference is tried to form to an opposite substrate side, it is necessary to pay an attention. In IPS system, to apply the effective in-plane switching, it is necessary to form, many thin and narrow shape inter digital electrodes, as a result many stepped differences are formed on the TFT side substrate. To dissolve the stepped differences it is effective to coat thick an organic high molecular layer, however when the thick insulation layer is formed on the electrode, it invites a lowering of the effective voltage which applies to the liquid crystal. As a result, a threshold voltage becomes high and it causes a problem in which since it is necessary to use driver having a high pressure withstand a consumption electric power becomes high. It is realistic to form the insulation layer having a proper thick to the TFT side substrate and to remain a proper stepped difference. As shown in FIG. 1, it is necessary to form a ratio a/d between the liquid crystal layer thickness d and the stepped difference a at least more than 0.02. In particular, for example, to drive the large size panel having diagonal size more than 18 inches, to restrain a deterioration of the voltage waveform from a driver LSI, it is necessary to lower the resistance value of the electrode, accordingly the electrode is necessarily to be formed thick. In the large scale panel, the stepped difference remains necessarily.
(2) Alignment Angle
In IPS-TFT-LCD, it is necessary to establish in principle an initial alignment direction to arrange a direction to which the electrode is extended, or by shifting with more than a constant angle from a vertical direction of the above stated direction. Herein, the electrode indicates a signal wiring electrode, a common electrode and a pixel electrode in a pixel. To regulate the initial alignment direction by the rubbing method, as stated in above, it is necessary to rub to a predetermined angle direction by the fibers having about 10-30 microns degree, by the stepped difference between the wiring being extended to the constant direction such as the signal wiring electrode, the common electrode in the pixel, and the pixel electrode in the pixel and the end portions thereof, it causes a problem in which the fibers are drawn into to the stepped difference direction from the design angle.
(3) Profound Degree of the Black Level
As one of the characteristics of IPS-TFT-LCD, it can enumerate that the profound of the black level (the black display) is good. Accordingly, the disturbance in the alignment comes to the fore in comparison with other systems.
In the conventional normally open type TN system, the dark level is obtained by the condition to which the high voltage is applied. In this case, under the high voltage almost liquid crystal molecular are all present to the electric field direction which is a vertical one direction to the substrate face, with the relationship between the liquid crystal molecular arrangement and a polarizing plate arrangement, the dark level can be obtained. Accordingly, the homogeneity of the dark level does not depend in principle on the initial alignment condition during the low voltage time. Further, the eyes of the human recognizes the roughness of the brightness as a relative ratio and further reacts near with a logarithms scale, accordingly it is sensitive to a fluctuation of the dark level. From the above stated view points, in the conventional normally open type TN system in which the liquid crystal molecular is arranged compulsively to one direction under a high voltage, it is insensitive to the initial alignment condition and it is in a better position than. On the other hand, in the in-plane switching system, to display the dark level at the low voltage or at the voltage of zero, it is sensitive to the disturbance of the initial alignment condition. In particular, an arrangement (it is called as a bi-refraction mode) in which a homogenous arrangement where the liquid crystal molecular alignment direction is in parallel each other on an upper substrate and a lower substrate is formed and also a light transmission axis of one polarizing plate is formed in parallel to the liquid crystal molecular alignment direction and that of another polarizing plate is formed in orthogonally, in a polarization light which is incident to the liquid crystal layer a linear polarizing light is propagated without the disturbance. This is effective to be profound in the dark level.
A transmission rate T of the bi-refraction mode is expressed by a following formula (1).
T=Toxc3x97sin2{2xcex8(E)}xc3x97sin2{Πxc3x97deff xc3x97xcex94n/xcex}.xe2x80x83xe2x80x83(1)
Herein, To is a coefficient and is a numeral value which is determined mainly by a transmission rate of the polarizing plate used in the liquid crystal panel, xcex8(E) is an angle making by the alignment direction (an effective light axis of the liquid crystal layer) of the liquid crystal molecular and the polarization light transmission axis, E is an application electric field strength, deff is an effective thickness of the liquid crystal layer, xcex94n is a refraction rate aeolotropic characteristic, and xcex is a wavelength of the light. Further, herein, a product the effective thickness deff of the liquid crystal layer by the refraction rate aeolotropic characteristic xcex94n, namely (deffxc3x97xcex94n), is called as a retardation. Further, the thickness deff of the liquid crystal layer is not a whole thickness of the liquid crystal layer but when the voltage is applied it indicates a thickness of the liquid crystal layer for changing practically the alignment direction. Because the liquid crystal molecular at a vicinity of an interface of the liquid crystal layer does not change the alignment direction when the voltage is applied according to an affect of an anchoring of the interface. Accordingly, when the thickness of the whole liquid crystal layer which is sandwiched by the substrates is expressed by dLC, between this thickness dLC and the thickness deff, it has always a relationship deff less than dLC and this difference differs from the liquid crystal material used in the liquid crystal panel, and the interface for contacting the liquid crystal layer, for example, the kinds of the alignment layer materials, and it can be estimated to have in general 20-40 nm degree.
As clearly understood from the above stated formula (1), the matter for depending to the electric field strength is sin2{2xcex8(E)} item and by changing angle xcex8 in response to the electric field strength E the brightness can be adjusted. To form the normally close type, under the voltage non-application time, since the polarizing plate is established to have xcex8=0 degree, it works to be sensitive to the disturbance of the initial alignment direction.
(4) Problems of the Photo-alignment Method
As explained in a latter portion, as to the problems from (1) to (3), it is possible to solve by an introduction of the photo-alignment method in place of the conventional rubbing method. The photo-alignment method is classified largely a photo-dissolution type and a photo-reaction type. In both cases, from a practical use there are following problems. To perform the close adhesion characteristic improvement between the alignment layer material and the substrate it is necessary to have some degree thickness, however in this case it is difficult to have the compatibility between the photo-reaction characteristic and the transparency characteristic. In a case the bad coloring is invited and the utilization efficiency of the light and the picture quality lower. The assurance between the close adhesion characteristic between the alignment control layer and the substrate is important in the practical use, and the success or the failure of the photo-alignment is essential.
Further, as stated in above, to adopt the photo-alignment to the substrate having the stepped difference structure, it is necessary to pay consideration to the following points. In the photo-alignment method, to a whole face of the alignment layer, the light having an ability for reforming a surface, for example as the ultra-violet light, is irradiated, in this case since the reflection of the light is caused at the stepped difference portion, it is necessary to pay consideration to the light course. It is desirable to have a modulated and inclined taper structure as the stepped difference. In a case when the ratio a/b of the parameter shown in FIG. 1 is less than 1 (in the inclination angle, less than 45 degree), the refection light at the taper portion is not reached to a pixel portion, accordingly it does not cause the alignment failure.
An object of the present invention is to provide in particular a large size liquid crystal display apparatus wherein the problem in which a manufacture margin is narrow in an alignment processing being a specific problem in IPS-TFT-LCD stated in above can be solved and a high quality picture quality in which a contrast ratio is heightened can be obtained and a generation of a display failure according to a fluctuation of an initial alignment direction can be reduced.
According to the present invention, in an active matrix type in-plane switching liquid crystal display apparatus in which at least one of a pair of substrates is transparent, a thickness of a liquid crystal layer is controlled to have a substantially constant thickness by a spacer which is dispersed and sandwiched between the pair of substrates, on a surface at a side for contacting at least one liquid crystal layer of the pair of substrates, an inclination degree a/b of a stepped difference portion having an alignment control layer which is formed on a surface of a display pixel is less than 1, and the alignment control layer is made by a material which is enable to give a liquid crystal alignment ability according to a polarization light radiation, and the liquid crystal alignment ability has at a vicinity of the stepped difference portion.
According to the present invention, during the formation of the alignment control layer of the panel having the stepped difference structure the manufacture margin can be enlarged and as a result it is possible to provide the large size liquid crystal display apparatus in which the initial alignment direction is formed homogeneously and the high quality picture quality can be obtained. In the conventional rubbing method, the end portion of the stepped difference works as a guide of the rubbing use cloth fibers and the fibers are drawn into the extension direction of the stepped difference and to the corner portions of the stepped difference the fibers do not reach, accordingly it is impossible to carry out the alignment processing and the alignment failure causes. On the other hand, according to the present invention, since the alignment ability is added by the polarization light radiation, in regardless the shapes the polarization light having the constant direction can be radiated to the minute portions, as a result the liquid crystal alignment direction is formed homogeneously. Further, the stepped difference structure can be formed at the substrate side having the electrodes group or at the opposed side substrate in which in commonly the color filter is formed. In particular, when the black matrix formed to the boundary portion of the color filter is made by the resin material and the black color paints, to make thicker this black matrix portion than the pixel portion in which the light is transmitted, the stepped difference is formed easily, and the present invention can work effectively. In reversely, when the black matrix is made by the metal thin layer material such as chromium, the black matrix portion become thin relatively. In this case, also the present invention can work effectively. Further, IPS liquid crystal, as clearly understood from the formula (1), since the liquid crystal layer thickness deff for giving the maximum transmission rate differs from every the waveform xcex of the light, and since the stepped difference is caused in a case where the liquid crystal thickness deff is changed at every color of the color filter, the present invention can work effectively. Further, when the dielectric rate aeolotropic characteristic is positive and also the angle making by the alignment direction of the electric field non-application time on the alignment control layer and the electric field direction is 45-88 degree and when the dielectric rate aeolotropic characteristic is negative and also the angle making by the alignment direction of the eclectic field non-application time on the alignment control layer and the electric field direction is 2-45 degree, in particular the present invention can work effectively.
Further, according to the present invention, in an active matrix type in-plane switching liquid crystal display apparatus in which between in a pair of substrates which at least one is transparent the liquid crystal layer is sandwiched, it is effective when the spacer having the regularity characteristic to the non-transparent portion of the pixel is formed in place of the dispersed spacer. In this spacer having the regularity characteristic can be formed, for example, according to a photo-lithography process. Naturally, since the spacer exists, it is necessary to form the stepped difference having the large size such as 2-5 microns degree and it is difficult to adopt the rubbing method.
Further, in particular it is necessary to pay the consideration about the problems of the close adhesion characteristic between the photo-reaction type alignment layer material and the substrate and the coloring characteristic. As to those problems, it can be solved by the intervene the transparent organic high molecular layer having the thickness thicker than the alignment control layer between the alignment control layer and the substrate. It is a desirable to have a value of 0.2-3 microns as the thickness of the transparent organic high molecular layer and it is a desirable to have a value of 0.01-0.1 microns as the thickness of the alignment control layer. The close adhesion characteristic is further strengthened by the addition of the adhesion promotion agent for heightening the close adhesion characteristic of the substrate face and the alignment control layer to the organic high molecular layer. Since the organic high molecular layer has some degree thickness, this works to reduce the deterioration caused by the temperature change etc. Further, more desirably when both of the alignment control layer and the transparent organic high molecular layer are made of polyimide, since they are the same system materials, the close adhesion characteristic can be further strengthened. When the transparent organic high molecular layer is made by amorphous material, since the whole face of the layer face presents homogeneous state, the reduction in the deterioration and the improvement in the close adhesion characteristic can be obtained. Further, to form thick the above stated transparent organic high molecular layer, polyamic acid having the concentration of more than 8% is coated, after that it is desirable to heat the temperature where the imization proceeds some degree, and to form thin the alignment control layer after soluble polyimide or polyamic acid has coated, it is desirable to heat the temperature where the solvent agent evaporates. In case of polyamic acid, it is desirable to rise to the temperature where the imization proceeds.